Numerous film deposition processes are known for depositing coatings or films onto the surfaces of various kinds of objects or substrates. One type of deposition process, known as chemical vapor deposition (CVD), is usually carried out in a vacuum chamber and in the presence of a reactive process gas, or a mixture of process gases, that are maintained under very low pressure. The material to be coated, usually referred to as the substrate, is placed in the vacuum chamber and is heated to a temperature sufficient to cause the reactive process gas (or mixture of gases) to react and deposit themselves on the surface of the substrate material. For example, the CVD process is widely used in the electronics industry to form various types of coatings, including silicon dioxide, silicon nitride, and polysilicon. While the CVD process carries with it the advantages of producing high quality surface coatings, with high densities, excellent adhesion, and fairly small microstructure, it suffers the disadvantage of being very expensive. Another disadvantage is that the deposition rate is very slow, on the order of 100 .ANG. or so per hour, and CVD cannot be used to produce coatings thicker than a few hundred angstroms.
Another kind of deposition process is known as thermal spraying or plasma spraying and is generally carried out at atmospheric pressure. In the thermal spraying process, the material to be deposited on the substrate is injected into a high temperature flame or plasma in a dry powder form. The high temperature plasma flame vaporizes a portion of the dry powder material, although a significant percentage of the material does not vaporize completely, but instead remains in the liquid state as small molten droplets. The liquid and vapor are then deposited on the substrate to form the surface coating. While the thermal spraying process has the advantages of being relatively inexpensive and can deposit coatings of virtually any thickness at high deposition rates, on the order of several tens of millimeters per hour, the presence of the molten droplets of material tends to significantly degrade the quality of the coating. Consequently, coatings deposited by thermal spraying tend to suffer in terms of adhesion, density, fracture toughness, hardness, composition homogeneity, uniformity, surface smoothness, and microstructure development.
Low pressure plasma spraying is essentially the same as thermal spraying, except that it is carried out under a partial vacuum or a "soft" vacuum. As a result, the low pressure plasma spraying process generally produces slightly higher quality surface coatings compared to atmospheric thermal spraying, while only suffering a slight reduction in deposition rate and the ability to produce thick coatings. Unfortunately, the need to carry out the process in a soft vacuum substantially increases the cost of low pressure plasma spraying, and the coating quality is not that much greater than is possible with conventional plasma or thermal spraying at atmospheric pressure.
While numerous other deposition processes are known and used, such as sputtering and vacuum deposition, such other deposition processes generally are not appropriate substitutes for the CVD or thermal spraying processes. For example, while coatings produced by sputtering are of excellent quality, with very high densities, extremely fine microstructure, and outstanding adhesion, the sputtering process must be performed under a "hard" vacuum and usually requires expensive and complex equipment. Also, sputtering technology has not yet developed to the point where it can produce certain types of coatings, such as certain oxide coatings, ceramic coatings, and cements. While the vacuum deposition process is considerably less expensive then sputtering, the deposition rates are extremely slow and coatings produced thereby are characterized by very poor adhesion. Also, the vacuum deposition process can only be used for certain types of coatings.
Consequently, there remains a need for a deposition process that can deposit relatively thick coatings at high rates, yet still produce high quality surface coatings in terms of adhesion, density, fracture toughness, hardness, composition homogeneity, uniformity, surface smoothness, and microstructure development. Put in other words, such a deposition process should achieve high quality surface coatings, comparable to the quality typically associated with chemical vapor deposition CVD processes, yet at the high deposition rates typically associated with plasma spraying processes. Additional advantages could be realized if such a deposition process would allow for the control of the oxidization/reduction state of the coating material, thus allow compound coatings to be produced. Still other advantages could be realized if the deposition process could be carried out at atmospheric pressure and with relatively inexpensive equipment.